In recent years, a microfabrication of a fabrication process rule has increasingly been developed by an enhancement in a semiconductor process technique. As a result, the number of transistors to be loaded onto one chip is increased and an SoC (System On a Chip) having a main function of a computer embedded in one chip has been used generally. Referring to the SoC, function blocks (circuit blocks) such as a microprocessor, an interface controller, a multimedia signal processing processor and a memory have been integrated into one chip, a necessary area for mounting is reduced, and a cost is also cut down more remarkably as compared with a system comprising a plurality of chips having an equivalent function. On the other hand, in the SoC in which a high integration progresses, there is a problem in that a consumed current is increased with an increase in the number of the transistors. The increase in the consumed current increases an amount of heat generation of the chip and deteriorates a reliability of the chip. In a portable apparatus, moreover, a battery driving time is reduced. An increase in the number of the transistors to be integrated into a chip and an increase in a leakage current of the transistor with a microfabrication mainly cause the increase in the consumed current. The type of the consumed current can be divided into two types including a DC current and an AC current. The DC current is a so-called leakage current which flows if a source voltage is supplied also in a state in which a circuit is not operated. On the other hand, the AC current is a charging/discharging current of the transistor and is a current (an active current) consumed when the circuit is operated. Conventionally, an attention has been paid to how to reduce the leakage current in order to reduce the consumed current. A method of reducing the leakage current in the SoC will be described below as an example.
The SoC is an aggregate of circuit blocks divided every function. In respect of an operating state of the SoC in a certain moment, all of the circuits are not operated. The reason is that the operation of only any of the circuit blocks to satisfy demanded functions is sufficient. By blocking the supply of a power to the circuit which is not used, therefore, it is possible to eliminate a leakage current of the circuit block. The method has been described in Patent Document 1, for example.
In order to carry out an analysis, a test or a feedback to a control, moreover, a technique for measuring an amount of a current in any of the circuit blocks is also required. They have been described in Patent Documents 2 and 3. The Patent Document 2 has described a method of providing switching means between a power wire and each of circuits or between a ground wire and the circuit, bringing the switch means into a blocking state and connecting a measuring apparatus between pads provided on both ends of the switching means, thereby measuring a consumed current in case of measuring the consumed current of each circuit. The Patent Document 3 has described that there are included switching means provided in each sub circuit and serving to block a current of the sub circuit, detecting means for detecting a power current of the sub circuit, and testing means for controlling the switching means in response to an output of the detecting means.
Recently, the function required for the SoC has been expanded and the number of the transistor operated in a certain moment has been increased. As a result, a current in the operation has also been increased so as not to be disregarded. In particular, there is given a demand for reducing a consumed current as greatly as possible in a portable apparatus. In the same manner as the leakage current, therefore, it is important to reduce the active current. Moreover, a method of measuring an amount of the active current consumed in any circuit block has also been important in the same manner.    [Patent Document 1] JP-A-2003-218682 Publication    [Patent Document 2] JP-A-63-93142 Publication    [Patent Document 3] JP-A-5-288798 Publication